1. Field of the Invention
The present invention relates to a back light device used for a liquid crystal display apparatus and the like, and a liquid crystal display apparatus using the same.
2. Related Arts
Recently, there has been widely used a liquid crystal display apparatus wherein a liquid crystal panel is illuminated from its back with a back light device, as various display apparatuses such as a display for a computer, or a television.
In such a liquid crystal display apparatus, polarized light obtained by causing light to transmit a polarizing plate is modulated in its liquid crystal layer.
For example, FIG. 18 illustrates a representative structure of a liquid crystal display apparatus comprising a conventional back light device.
In this liquid crystal display apparatus 1, the back light device 2 causes light outputted from a light source 3 to be incident onto one side end face 4A of a lightconductor 4 in a substantially plate form, and to be outputted from a light outputting surface 4B (the upper surface in FIG. 18), and then the outputted light for the light outputting surface 4B is diffused with a diffusing sheet 5 and converged in a prism sheet 6 to illuminate a liquid crystal cell 7 from its back.
The light made incident onto the side end face 4A, from the light source, goes forwards inside the lightconductor 4 with being totally reflected therein, and the advancing direction of a part of the light is changed by means of light diffusing spots 4D arranged on a back face 4C to be outputted from the light outputting surface 4B to the outside the lightconductor 4. The light outputted downward from the back surface 4C (the lower surface in FIG. 18) opposite to the light outputting surface 4B of the lightconductor 4 is reflected on a reflecting sheet 8 to be again returned into the lightconductor 4.
The prism sheet 6 is a sheet on which triangle pole prisms are arranged in such a form that their ridge lines are in parallel, and the liquid crystal cell 7 has structure wherein its front and back faces are sandwiched by polarizing plates 9A and 9B.
FIGS. 19 and 20 illustrate other conventional liquid crystal display apparatuses, respectively. The liquid crystal display apparatus 1A shown in FIG. 19 is different from the apparatus shown in FIG. 18 in that a diffusing sheet 5 and a prism sheet 6A in a back light device 2A are arranged oppositely to those in the back light device 2 shown in FIG. 18, and that the triangle pole prisms on the prism sheet 6A are arranged to be directed downward, that is, oppositely to the prisms shown in FIG. 18. Other structure is the same as in the apparatus shown in FIG. 18. Thus, the same reference numbers are attached to the same elements, and explanation thereof is omitted.
In a back light device 2B in the liquid crystal display apparatus 1B shown in FIG. 20, between the diffusing sheet 5 and the prism sheet 6A in the back light device 2A shown in FIG. 19, arranged is the third prism sheet 6B wherein the direction of its triangle pole prisms is perpendicular to the prism sheet 6A. Other structure is the same as in the liquid crystal display apparatus 1A shown in FIG. 19.
In all of the display apparatuses 1, 1A and 1B, the liquid crystal cell 7 is sandwiched by the polarizing plates 9A and 9B, and the polarizing plates 9A and 9B absorb about 50% of the incident light beam so that the efficiency for using the light is low. Thus, in order to obtain sufficient brightness, it is necessary that a greater deal of light is made incident onto the polarizing plate. In this case, however, various problems are caused. For example, power consumption for the light source increases, and heat from the light source has a bad influence on the liquid crystal in the liquid crystal cell 7 so that it is difficult to watch the display.
In order to overcome the above-mentioned problems, the following prior art inventions for improving the efficiency for using light are proposed in, for example, Japanese Patent Application Laid-Open (JP-A-) No. 4-184429, 6-265892, and 7-261122: A polarized beam splitting sheet is used to separate non-polarized light from a light source into two linearly polarized light beams which are perpendicular to each other, and then the one of the polarized light beams obtained from the separation is made incident onto a liquid crystal cell, and the other, which is a parallel component, is returned to the light source side so as to be again introduced into the polarized beam splitting sheet, for use, by reflection and the like.
In the invention disclosed in JP-A-No. 4-184429, non-polarized light from a light source is divided into two polarized beams which are perpendicular to each other with a polarized beam splitter, and then the one polarized light is outputted directly toward a liquid crystal, and the other polarized light is returned to the light source side to be converged, followed by being reflected to be again made incident onto the polarized beam splitter.
In the invention disclosed in JP-A-No. 6-265892, a light deflector comprising a prism array is disposed at the light outputting surface of a planar lightconductor so that the outputted light will be substantially perpendicular to the surface of this planer lightconductor, and further at the light outputting surface side, a polarized beam splitting means is disposed which comprises a polarized beam splitting layer deposited on the inclined plane of the prism array, for example in a triangle pole form.
In the invention disclosed in JP-A-No. 7-261122, a polarized beam splitting sheet is disposed at the light outputting surface side of an element for producing parallel luminous flux, comprising a light scattering lightconductor including a portion having a cross sectional shape of a wedge.
The invention disclosed in JP-A-No. 4-184429 is applied to a projection type liquid crystal display apparatus, and an illuminating device therein requires a large space. This cannot be applied to a plane type liquid crystal display apparatus, wherein a back light device is located adjacently to the back face of a liquid crystal panel.
The invention disclosed in JP-A-No. 6-265892 is applied to a thin liquid crystal display apparatus. However, the structure of the polarized beam splitting means comprising a triangle pole prism array is complicated. It is especially difficult to form the polarized beam splitting layer on the inclined plane of the triangle pole prism array. Thus, there remains a problem that mass production thereof is difficult.
In the invention disclosed in JP-A-No. 7-261122, the element for producing parallel luminous flux, comprising, as a lightconductor, a specific light scattering lightconductor having a cross sectional shape of a wedge is used and consequently there remains a problem that it is difficult to control the light scattering lightconductor so as to have a specific light scattering ability.
In the light of the aforementioned problems, an object of the present invention is to provide a back light device which can be made thin by using a relatively simple structure and which has excellent mass producibility and improved efficiency for using light; and a liquid crystal display apparatus using the back light device.